The objective of this project is to determine the relative involvement of each of four lower brain stem structures in the mediation of facial pain. These are: (1) the ventromedial border region of the trigeminal nucleus caudalis including the immediately subjacent lateral reticular formation, (2) the zona marginalis of the nucleus caudalis, (3) rostral medullary and pontine extensions of the lateral reticular formation and (4) the central core of the bulbar reticular formation. Two concurrent programs of research will be performed. One will consist of acute electrophysiological experiments, principally with the cat, in which graded, mechanical and electrical, noxious and non-noxious stimuli will be applied to orofacial fields. Unit activity recorded extracellularly from responsive neurons in each of the four brain stem structures will be analyzed and quantified. Following histological verification of recording sites, data obtained from the four structures will be compared. The other program will make use of two groups of waking, unanesthetized monkeys with chronically implanted dental electrodes. Operant procedures will be used to obtain behavioral measures of pain induced by systematic, highly controlled stimulation of the tooth pulp through the electrodes. The measures will include escape thresholds and force of manual escape responses. Alterations in performance produced by discrete lesioning of the four brain stem structures will be studied in one of the groups of monkeys. Unit discharges evoked by pulpal stimulation will be recorded from brain stem neurons of the other group of animals through indwelling, movable microelectrodes. Behavioral, anatomical and physiological data obtained from the two groups will be analyzed and compared. Ultimately the data will be used to provide a meaningful context for the interpretation of the results of our acute electrophysiological experiments.